1. Field of the Invention
The invention relates to the field of 3-phase power factor corrected rectifiers, active power filters, and grid-connected inverters and in particular to control methods based on one-cycle control.
2. Description of the Prior Art
The invention in this document covers vast applications spanning from power factor corrected rectifiers in front of and active power filters in parallel to electronic equipement such as computers, communication, motion control, aviation, space electronics, etc. to the grid-connected inverters for distributed power generation.
Power factor corrected rectifiers
In recent years, the usage of modern electronics equipment has been widely proliferating. The electronics equipment usually have a rectifier of single-phase or three-phases in the front end. Three-phases are more desirable for high power applications. A three-phase rectifier is a device that converts three-phase sinusoidal ac power into dc power. Traditional rectifiers draw pulsed currrent from the ac main as shown in FIG. 1, which causes significant harmonic pollution, low power factor, reduced transmition efficiency, harmful electromagnetic interference to neighborhood appliances, as well as overheating of transformers.
In order to solve these problems, many international agencies have proposed harmonic restrictions to electronic equipment. As a result, a vast number of power factor corrected (PFC) rectifers have been proposed to comply with these regulations.
A three-phase power factor corrected rectifeir is a device that converts three-phase sinusoidal ac power into dc power while the input currents are sinusoidal and unity power factor, as shown in FIG. 2. Many three-phase topologies are suitable for implementing PFC function for rectification. Usually, high frequency active switches are used in the rectifiers to realize the PFC function.
The control methods that modulate the pulse width of the switches are an important issue in the power electronics research. A third harmonic injection method was reported for a dual-boost converter with center-tapped dc-link and split output capacitors. This method achieves low current distortion. However, it is not convenient to generate the third harmonic signal tuned to the right frequency and right amplitude.
Hysteresis control and d-q transformation control were frequently used control approaches. Hysteresis control results in variable switching frequency that is difficult for EMI filter design. The d-q approach is based on digital implementation that leads to complicated systems. An analog control method with constant switching frequency modulation was reported for a particular rectifier, where several multipliers are necessary to implement the three phase current references. Due to the disadvantages of variable frequency or complexity of implementation, three-phase PFC rectifiers are not commercially practical.
Active power filters:
One alternative for dealing with the current harmonics generated by treaditional rectifiers is to use active power filters (APF). Considering electronic equipment with traditional rectifier as nonlinear loads to the ac main, a three-phase APF is a device that is connected in parallel to and cancels the reactive and harmonic currents from one or a group of nonlinear loads 110 so that the resulting total current drawn from the ac main is sinusoidal as shown in FIG. 3. In contrast to PFC, where a PFC unit is usually inserted in the energy pass, which processes all the power and corrects the current to unity power factor, APF provides only the harmonic and reactive power to cancel the one generated by the nonlinear loads. In this case, only a small portion of the energy is processed, which may result in overall higher energy efficiency and higher power processing capability.
Most APF control methods proposed previously need to sense the three-phase line voltages and the three-phase nonlinear load currents, and then manipulate the information from these sensors to generate three-phase current references for the APF. Since the reference currents have to reflect the load power of the nonlinear load, several multipliers are needed to scale the magnitude of the current references. A control loop is necessary to control the inverter to generate the reactive and harmonic current required by the nonlinear load. These functions are generally realized by a digital signal processing (DSP) chip with fast analog-to-digital (A/D) converters and high-speed calculations. The complex circuitry results in high cost and unreliable systems, preventing this technique from being used in practical applications.
Some approaches that sense the main line current were reported for single-phase APF and for three-phase APF. The overall circuitry is reduced. However, multipliers, input voltage sensors are still necessary. High speed DSPs are still used in three-phase systems due to the complexity of the systems.
Grid-connected inverters:
Distributed power generation is the trend in the future in order to promote new power generation technologies and reduce transmition costs. An effective use of natural resources and renewable resources as alternatives to fossil and nuclear energy for generation of electricity has the effect of protecting the environment. In order for the alternative energy sources to impact the energy supply in the future, they need to be connected to the utility grid. Therefore, grid-connected inverters are the key elements for the distributed power generation systems. A grid-connected inverter is a device that converts dc power to ac power of single phase or three-phase power that is injected to the utility grid. In order for an alternative energy source to be qualified as a supplier, sinusoidal current injection is required as shown in FIG. 4.
Again, control methods are crucial. In the past, d-q transformer modulation based on digital implementation was often employed for a standard six-switched bridge inverter topology. The complexity results in low reliability and high cost. In addition, short-through hazard exists in this inverter.
What is needed is a design for 3-phase power factor corrected rectifiers, active power filters, and grid-connected inverters which overcomes each of the foregoing limitations of the prior art.
The method of the invention is an unified constant-frequency integration (UCI) control method based on one-cycle control. It employs an integrator with reset as its core component along with logic and linear components to control the pulse width of a three-phase recitifier, active power filter, or grid-connected inverter so that the all three phase current draw from or the current output to the utility line is sinusoidal. No multipliers are required, as used in many control approaches to scale the current references according to the load level. Furthermore, no reference calculation circuitry is needed for controlling active power filters.
The UCI control employs constant switching frequency and operates in continuous conduction mode (CCM) that is desirable for industry applications. This control approach is simple, general, and flexible and is applicable to many topologies with slight modification of the logic circuits, while the control core remains unchanged. Although, a DSP is not required to implement the UCI control; if in some cases a DSP is desired for other purposes, the unified constant-frequency control function may be realized by a low cost DSP with a high reliability, because no high speed calcutation, high speed A/D converter, or mutipliers are required.
The implementation of UCI control can be roughly classified into two categories: (1) vector control mode and (2) bipolar control mode. A three-phase system in the vector control mode has only two switches operating at a switching frequency at a given time, while a three-phase system in bipolar control mode has three switches operating at a switching frequency.
Power factor corrected rectifiers:
The power train of a three-phase rectifier is usually a boost-derived three-phase converter. The UCI controller for PFC applications can control the power train in either the vector and bipolar control mode.
In vector control mode, the boost-derived three-phase rectifiers are categorized into two groups. One group of them can be decoupled into a series-connected dual-boost topology that features central-tapped or split dc output capacitors. The other group can be decoupled into a parallel-connected dual-boost topology that features a single dc output capacitor. The dual-boost sub-topologies rotate their connection every 60xc2x0 of the line cycle depending on the line voltage states: In each 60xc2x0 of ac line cycle, only two switches are switched at high frequency. Therefore, the switching loss is significantly reduced. The switches operate at a current lower than the phase current, which results in reduced current ratings and conduction losses.
In bipolar control mode the input phase voltage, output dc voltage, and duty ratios of switches for some boost rectifiers are related by a general equation. Based on the one-cycle control and solutions of this general equation, which is singular and has infinite solutions, several UCI control solutions are derived below for topologies such as the standard six-switch rectifier, VIENNA rectifier and similar circuits.
The UCI control method for PFC rectifiers has demonstrated excellent performance, great simplicity, namely an order of magnitude fewer components than prior art, and unparallelled reliability.
Active Power Filters:
The UCI control approach is based on an one-cycle control and senses main currents. No multipliers nor reference calculation circuitry are required. Thus, the implementaion cirucitry is an order of magnitute simpler than previously proposed control methods. The UCI control for active power filters has serveral solutions. One solution uses a vector control mode and the need to sense the three-phase line voltage. The switching losses for vector control are reduced. The other version uses bipolar control mode that eliminates three-phase line voltage sensors.
Inverters for Alternative Energy Sources
Grid-connected inverters with sinusoidal current output are proposed. A three-phase standard bridge inverter can be decoupled in a parallel-connected dual-buck subtopology during each 60xc2x0 of line cycle. Therefore, only two switches are controlled at switching frequency in order to realize sinusoidal output with unity-power-factor. By the proposed control method, the grid-connected inverter features an unity-power-factor, low current distortion, as well as low switching losses. In addition, short-through hazard is eliminated because only one of switches in each bridge arm is controlled during each 60xc2x0 of line cycle.
While the method has been described for the sake of grammatical fluidity as steps, it is to be expressly understood that the claims are not to be construed as limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations under 35 USC 112, but to be accorded the full scope of the meaning and equivalents of the definition provided by the claims. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.